Podophyllotoxin is a naturally occurring compound extracted from the mandrake plant. Recently a therapeutically useful semi-synthetic glycoside of podophyllotoxin, etoposide (also known as VP-16), shown below, has been developed. ##STR2##
This compound exhibits therapeutic activity in several human neoplasms, including small cell carcinomas of the lung, testicular carcinomas, Hodgkin's disease, leukemia, lymphoma and Kaposi's Sarcoma.
It is believed that these drugs block the catalytic activity of DNA topoisomerase II by stabilizing an enzyme-DNA complex in which the DNA is cleaved and covalently linked to the enzyme (Chen, Ross, Rowe), which are all herein specifically incorporated by reference. By way of background, topoisomerases are enzymes which control the topological state of DNA. Type II topoisomerases catalyze DNA strand passage through transient double strand breaks in the DNA. The resulting change in the linking number of DNA allows these enzymes to mediate DNA interconversions, such as supercoiling and relaxation of supercoiling, catenation and decatenation, knotting, and unknotting (Wang, 1985, Maxwell).
Type II DNA topoisomerase enzymes have been shown to be involved in a number of vital cellular processes, including DNA replication and transcription, and chromosomal segregation. These enzymes, therefore, are a critical target for the action of a wide variety of anticancer drugs, including etoposide. The key step leading to cell death may be the capability of these drugs to block the catalytic activity of DNA topoisomerase II, as noted above.
Structure-activity studies have demonstrated a direct correlation between cytotoxicity, DNA breakage, and murine-derived topoisomerase II inhibition activities among the podophyllotoxin analogues (Minocha). The isolation and purification of human type II topoisomerase from lymphocytic leukemia cells has provided the means to use this enzyme as a target to investigate the structure-related activity relationships among etoposide and related congeners.
It has been shown that the substitution of etoposide's glycosidic moiety by an 4-alkoxy group, as in 4'-demethyl-epipodophyllotoxin ethyl ether, preserves the inhibitory activity of DNA topoisomerase II intact at higher concentrations (Thurston, 1986). However, it has also been shown that a series of 4-acyl congeners are less active, even though some of them possessed potent cytotoxicity (Thurston, 1988).
Although etoposide has been widely used at the clinical level, the development of drug resistance, myelosuppression, and poor oral bioavailability has encouraged synthesis of analogues related to etoposide which possess preferred pharmacological profiles. Previous studies by the inventors were directed at substituted amino analogs. These analogs are disclosed in U.S. patent application 07/313,826, filed Feb. 23, 1989, hereby incorporated by reference. These compounds have also been disclosed by the applicants in the literature (Wang, 1990a, 1990a), and these references are also incorporated herein by reference. The compounds described therein have yielded numerous useful compositions which can be converted to water soluble products. Not only are many of these compounds more potent than etoposide in the inhibition of human DNA topoisomerase II and in causing protein linked DNA breakage, but these compounds also display activity against KB resistant cells.
Other etoposide analogs which posses anti-cancer activity have been disclosed in Japanese patent No. H1-197486 (August 9, 1989). The Japanese patent discloses compounds of the following formula: ##STR3## wherein R is a sugar moiety selected from arabinosyl, xyrosyl, rhamnosyl, glucosyl, and 4,6-ethylene glucosyl. This patent also discloses a synthetic method for the intermediate of the formula: ##STR4##
Improved methods for the production of this compound have been disclosed (e.g., Lee).
Another podophyllotoxin derivative synthesized in the art is 3',4'-didemethoxy-3',4'-dioxopodophyllotoxin of formula: ##STR5## Ayers et al. (1980) disclosed the synthesis of this compound by reacting podophyllotoxin with nitric acid. Nemec discloses a similar oxidation of etoposide-3',4'-orthoquinone, and related compounds, in U.S. Pat. No. 4,609,664 using sodium periodate as an oxidizing agent.